Are all eggs created equal? Food availability and the fitness trade-off between reproduction and immunity

Summary 1. Reproduction and immune function are critical processes, but organisms can rarely optimize both traits. Resultant reproduction–immunity trade-offs may be ‘facultative’, occurring only when resources are scarce, or they may be ‘obligate’, occurring regardless of resource availability. 2. Previous research has tested for the ‘facultative’ or ‘obligate’ nature of reproduction–immunity trade-offs by measuring resource allocation (e.g. follicle size). However, measuring resource allocation alone may be insufficient when gauging the fitness consequences of reproduction–immunity trade-offs because the number and quality of eggs or offspring trade off with one another. 3. We used the Texas field cricket (Gryllus texensis) to provide the most direct test to date of whether a fitness trade-off between these two traits is ‘facultative’ or ‘obligate’. We used a factorial design to manipulate food availability and immune status throughout adulthood. We then estimated lifetime fecundity, hatching success and their product (reproductive success), and we also measured several aspects of offspring quality (e.g. egg size and protein content, and hatchling size and energy stores). 4. A reproduction–immunity trade-off was ‘obligate’ in this species because immune challenge reduced reproductive success estimates regardless of food availability. Females with unlimited food were more fecund and produced more and larger hatchlings, but neither food availability nor immune status affected egg size, egg phenoloxidase activity, incubation duration, hatching success or hatchling energy stores. We observed a trade-off between offspring size and number – females favouring offspring size over fecundity produced fewer hatchlings, but their hatchlings were of higher quality (larger and more robust). 5. By demonstrating that not all eggs are created equal, we provide key insight into the role of reproductive allocation in the fitness trade-off between reproduction and immunity.